DOI: 10.1148/radiol.2373051151
(Radiology 2005;237:755-756.)
© RSNA, 2005
Automatic Exposure Control in CT: Are We Done Yet?
Cynthia H. McCollough, PhD
1 Department of Radiology, Mayo Clinic College of Medicine, 200 First St SW, Rochester, MN 55905, (email: mccollough.cynthia{at}mayo.edu)
The Setting
The tremendous growth in computed tomographic (CT) applications and utilization over the past decade has resulted in considerable emphasis being placed on the need to adjust technique factors based on patient size (ie, attenuation), primarily by a global adaptation of the tube current (1). It is also possible to modulate the tube current within a scan to further reduce dose (2,3). The modulation may be fully preprogrammed, occur in near-real time by using a feedback mechanism, or incorporate preprogramming and a feedback loop; it can also occur angularly about or along the long axis of the patient. These methods of adapting the tube current to patient attenuation, known generically as automatic exposure control, are analogous to photo-timing in general radiography and have demonstrated reductions in dose of about 20%–40%. In this issue of Radiology, Funama et al (4) report results of their evaluation of the effect of tube voltage on radiation dose and demonstrate that tube voltage, as well as tube current, needs to be adapted to patient size.
The Science
Funama et al (4) found that a reduction in tube voltage from 120 kV to 90 kV helps to reduce the radiation dose by as much as 35% without sacrificing low-contrast detectability. Similar results have been reported by Huda et al (5) and Siegel et al (6). The underlying physics principle is relatively straightforward: Iodine attenuation increases as tube potential (tube voltage) decreases because the energies in the x-ray beam move closer to the k-absorption edge of iodine. This beneficial effect, however, is offset by an overall increase in tissue attenuation as tube voltage is decreased, thus decreasing the amount of transmitted x-rays and increasing image noise. Hence, decreasing tube voltage leads to an increase in iodine contrast and an increase in image noise. The question then is which effect dominates, and at which tube voltage is the dose minimized for a specific patient size and desired contrast-to-noise ratio.
Funama et al (4) present a thorough analysis in which they compare an image of a phantom with low-contrast resolution acquired at 120 kV with images of the same phantom acquired at 90 kV and various tube current–time product settings. They measured the contrast-to-noise ratio as an indicator of low-contrast performance and combined these results with human observer performance data by using the same low-contrast phantom. The receiver operating characteristic curves showed that, at the lower tube voltage, observers detected low-contrast objects equally as well as at 120 kV, providing that the tube current–time product was increased enough to sufficiently compensate for the increased noise level. Dose measurements at the respective techniques were performed to determine which of the options delivered the lower dose (low tube voltage and higher tube current–time product or high tube voltage and lower tube current–time product). It was found that the techniques with low tube voltage and higher tube current–time product delivered the lower dose.
The Practice
Clinical use.—The implications for further dose reduction in CT are clear. For small patients, the capabilities of current CT systems allow the higher tube current values that are needed with lower tube voltage settings. Thus, with no investment in new technology, the adoption of protocols with lower tube voltage—and the resultant dose reduction—can be immediately accomplished in body CT imaging of children and small adults. For CT of the head, however, the clinical acceptance of lower tube voltage settings will likely need to wait until manufacturers implement appropriate modifications to existing beam-hardening corrections.
Future opportunities and challenges.—In small patients, the selection of techniques with low tube voltage and higher tube current–time product provides another important method for reducing the dose in CT, additive to the dose reduction achieved with existing tube current modulation schemes. Future implementations of automatic exposure control systems need to include this approach—first determining the lowest tube voltage setting that is consistent with the patient's attenuation and the examination indication, and then following the same steps for optimizing tube current, as in present automatic exposure control systems.
It is essential that users understand that currently available automatic exposure control systems do not include this tube voltage adaptation step. The reference tube current–time product or noise index that is appropriate for an examination performed at 120 kV will not be correct for an examination performed at 80 or 90 kV. Thus, the greatest challenge to the adoption of techniques with low tube voltage will be the current lack of guidance in regard to the appropriate tube current–time product for a given tube voltage and patient size. Without help from a medical physicist or the manufacturer to establish tube voltage and tube current–time product values for specific patient sizes, technologists may struggle with the transition to the use of a lower tube voltage. It will not suffice to simply request that technologists begin scanning at a lower tube voltage. If they do not know the correct amount by which to increase the tube current–time product, the scan will be either of poor quality (too low a tube current–time product) or of higher dose than necessary (too high a tube current–time product). The selection of the appropriate tube current–time product should not be left to chance.
Summary
It is clear that there is great potential for further dose reduction in CT. It is time to move this science into practice, as did Nakayama et al (7) in a companion article published in the same issue of Radiology. The authors performed abdominal CT in 40 patients at 120 kV, with follow-up scanning performed at 90 kV. Although they did not increase the tube current–time product at the lower tube voltage setting, overall image quality and enhancement of abdominal organs were not significantly different between the examinations performed at 120 kV and at 90 kV (P > .5). What was different was dose, with a weighted CT dose index reduction of 56.8% at the lower tube voltage. Clearly, the radiology community is not done with dose reduction in CT until radiologists adopt the principles of scanning with low tube voltage for small patients into clinical practice.
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Related Article
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Radiation Dose Reduction without Degradation of Low-Contrast Detectability at Abdominal Multisection CT with a Low–Tube Voltage Technique: Phantom Study
- Yoshinori Funama, Kazuo Awai, Yoshiharu Nakayama, Kiyotaka Kakei, Nozomu Nagasue, Masamichi Shimamura, Natsuko Sato, Shamima Sultana, Shoji Morishita, and Yasuyuki Yamashita
Radiology 2005 237: 905-910.
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